void merge_sort(int q[], int l, int r)
{
    if (l >= r)
        return;

    int mid = l + r >> 1;
    merge_sort(q, l, mid);
    merge_sort(q, mid + 1, r);

    int k = 0, i = l, j = mid + 1;
    while (i <= mid && j <= r)
        if (q[i] < q[j])
            tmp[k++] = q[i++];
        else
            tmp[k++] = q[j++];

    while (i <= mid)
        tmp[k++] = q[i++];
    while (j <= r)
        tmp[k++] = q[j++];

    for (i = l, j = 0; i <= r; i++, j++)
        q[i] = tmp[j];
}